Many contemporary implantable stimulation devices, such as pacemakers and ICDs, are typically designed to respond, in some manner, to tachyarrhythmias—which are essentially very fast heart rhythms. Stimulation devices are also designed with so-called refractory periods. A “refractory period” refers to an interval or timing cycle following a sensed or paced event during which the device's sense amplifier will not respond to incoming signals. Dual-chamber devices have separate refractory periods for each chamber (atrial and ventricular). The refractory period is designed into the stimulation devices to preclude the devices from responding to normal, but inappropriate signals (e.g. T-waves, evoked responses, etc.). The normal refractory periods, however, can preclude sensing of the very fast rates.
The refractory period can be segmented into two segments—the absolute refractory period and the relative refractory period. For a stimulation device, the absolute refractory period refers to the time period directly following a sensed or paced event during which all activity is ignored by the device's sense amplifier. The absolute refractory period is followed by the relative refractory period which refers to a “noise-sampling” or other detection portion during which sensing occurs, but the information is used for purposes other than resetting timing cycles. Examples of such purposes include atrial rate detection of Automatic Mode Switching and differentiation of a conducted atrial premature beat from a true PVC. Automatic Mode Switching or “AMS” refers to an algorithm which causes the pacemaker to revert from a tracking mode (e.g. DDD or VDD) to a non-tracking mode (e.g. DDI or VVI) upon recognition of a pathologic rapid intrinsic atrial rhythm. In order for the pacemaker (or pacemaker component of an ICD) to detect very rapid atrial rates that would normally be obscured or hidden by the normal PVARP (post-ventricular atrial refractory period) timing cycle, the current generation devices utilize a microprocessor to monitor events occurring within the PVARP. This allows recognition of very rapid atrial rhythms and engagement of AMS.
Accordingly, during the relative refractory period, the device can look to see if there are events that occur (e.g., fast atrial or ventricular rates) for which therapy or diagnosis is needed. There are certain events that can occur, however, that are otherwise normal, but which should not be counted for purposes of assessing whether there is a fast rate. For example, it is not uncommon to sense a ventricular depolarization (QRS complex) up in a lead positioned in the atrium. Yet if the device mistakenly counts this complex as an atrial event, the device may mistakenly determine that the patient is experiencing a rapid atrial rate. This can lead the device to provide erroneous diagnoses or administer inappropriate therapy.
Expanding on the previous example. A common physiologic but inappropriate signal that is detected is the far field R wave (FFRW). A FFRW is a ventricular depolarization that is also detected on the atrial channel. Commonly, the FFRW follows the sensed or paced ventricular event. Sensing the ventricular event initiates a timing circuit on the atrial channel designated the Post-Ventricular Atrial Blanking (PVAB) period. Recall that a blanking period is an interval during which the pacemaker is absolutely refractory—that is, it not capable of detecting any event. Hence, the standard method for managing FFRW signals is to program the PVAB to a sufficient duration to preclude detection of a FFRW if present.
Consider that one could arbitrarily program a very long PVAB, however, any period of blanking effectively blinds the pacemaker to appropriate events occurring during that interval and will, in the case of the AMS algorithm, either delay the recognition of an atrial tachyarrhythmia or totally prevent its recognition. Hence, one would prefer to keep the PVAB as short as possible.
A paper presented at Europace 2001, on 25 Jun. 2001, demonstrated a difference between the timing of FFRWs associated with either a native or paced ventricular complex. The paper was entitled Closer investigation of oversensing: sense amplifier signal analysis, Europace 2001; 2: Suppl B: B146 (abstract 454). In this paper, the interval between the sensed R wave and the FFRW detected on the atrial channel is termed the far field R wave duration (FFRD). In response to a sensed native R wave, the FFRD was 25 ms (bipolar) and 49 ms (unipolar). When one examined paced ventricular events, the FFRD with the paced R wave was 83 ms (bipolar) and 150 ms (unipolar).
A second paper also presented at Europace 2001, on 26 Jun. 2001, entitled Overdrive pacing for atrial fibrillation—complications and ways to overcome them, Europace 2001; 2: Suppl B: B203 (abstract 648), focused on a series of patients implanted with the Vitatron Selection® 900 pacemaker. It was noted that the FFRW between the detected native ventricular R wave and the detected atrial signal was relatively short, but since the PVAB had been programmed to an even shorter interval, inappropriate AMS occurred because the system interpreted the FFRW as a true P wave. When this was recognized by the clinicians, they increased the PVAB which, in turn, prevented AMS in response to FFRW associated with native R waves. The patient then developed some level of AV block such that the ventricular events were all paced. The patient again began experiencing AMS episodes shown to be inappropriate and due to FFRW sensing. The interval from the ventricular stimulus to the FFRW signal associated with ventricular pacing was 100 ms longer that the interval from the detected R wave to the FFRW signal. When the PVAB was increased further to preclude these inappropriate signals, inappropriate AMS episodes were prevented. However, now the system was utilizing a very long PVAB that compromised recognition of native atrial tachyarrhythmias.
In a second example, a biventricular ICD may detect depolarizations in both the right ventricle and the left ventricle in response to a single ventricular depolarization. This will result in double counting and the misidentification of a normal heart rhythm as a pathologic tachycardia resulting in the delivery of therapy that is inappropriate for the actual rhythm.
It would be desirable to protect against detecting inappropriate signals. The implantable stimulation device should certainly respond to appropriate rhythms, but the device should also avoid false triggering and/or false diagnostics.
Accordingly, this invention arose out of concerns associated with providing improved methods and systems for reducing detection of inappropriate physiologic signals.